The long term objective of this project is to increase knowledge of the age-dependent changes in auditory responsiveness that occur early in life. The first goal of this study is to investigate the development of frequency resolution in the human auditory system. The primary means of assessing frequency resolution will be through the use of pure-tone masked tuning curves constructed using transient-evoked otoacoustic emissions (TEOAE) and waves I and V of the auditory brainstem response (ABR). Experiments will test the hypothesis that frequency resolution is more "adult-like" at the peripheral level (TEOAE and ABR wave I) compared to a central level (ABR wave V). The second goal is to identify those mechanisms which underlie age- related change in physiologic responses to auditory stimuli early in life. Simultaneous development of the external- and middle-ear, sensory apparatus, and neural pathway likely contribute to the changes observed in infant responses to sound. Addressing the influence of these structures in normal-developing infants is confounded by the fact that the development of different structures likely occurs simultaneously and may be highly correlated. One way to address specific elements of this developmental course is to examine an auditory system that has known disorders. To this end we will test the hypothesis that inherent auditory system abnormalities in infants with Down syndrome will influence physiologic measures from the middle ear, cochlea, and neural pathway. Assessing the performance of physiologic and acoustic tools in these infants will extend our knowledge of these tools, as well as the underlying auditory system itself. The characteristics of physiologic responses -- ABR, otoacoustic emissions, and acoustic measures of middle- ear input impedance -- will be determined in infants with Down syndrome. Two sets of experiments are proposed. The first addresses 1) whether properties of the infant external and middle ear influence ABR threshold, and 2) whether change in frequency resolution across age is determined by peripheral mechanisms or central maturation in the auditory pathway. The experiments in the second set are designed to characterize inherent anatomical abnormalities in infants with Down syndrome using physiologic measures of cochlear function and middle-ear impedance.